DE69327644T2 - Remote controlled security system - Google Patents

Description

The present invention relates generally to a remote controlled security system and more particularly to a security system using synchronized one-way transmissions in which synchronization between transmitter and receiver is maintained even after inadvertent actuations of the transmitter. Many security systems are available in which a push-button remote transmitter enables a function in the vehicle, such as unlocking the doors or turning off an alarm. Such systems may have the remote control unit broadcast a fixed identification code that is unique to the transmitter/receiver combination. However, a security system using a fixed code can be beaten by using a random scanning device that sends random codes in rapid succession in the hope of reproducing a valid code. Security in such systems is increased by increasing the number of combinations in the security code, i.e., by lengthening it.

The fixed code system can also be beaten by using a recording device that intercepts and then duplicates transmissions of the security code from a valid transmitter.

There are other security systems known which can detect a Use a security code that alters the transmission, thereby eliminating the possibility of a transmission being intercepted and retransmitted because an exact copy of the previously transmitted signal is not accepted by the receiver.

One known method of changing the security code uses a table in both the transmitter and receiver which is indexed each time a transmission is made. A problem with such a system is that if the transmitter is operated out of range of the receiver, the transmitter will no longer be synchronized with the receiver - and will no longer be able to control the system. To alleviate this synchronization problem, two-way transmitters have been used which use a handshaking technique to ensure that the transmitter and receiver are indexed simultaneously. However, a need for both transmit and receive capabilities in the remote control unit and the device in the vehicle is undesirable.

A security system described in U.S. Patent 4,847,614 uses a one-way transmission method. A digital code is stored in system memory and is changed in both the transmitter and receiver using digital logic gates that are activated by the bits to generate the next code in sequence after each transmission. This system also suffers from synchronization problems which, if severe enough, will result in the operator using a key to manually unlock the door. If the system is less than 10 steps out of synchronization, the system will automatically resynchronize. If the system is 10 to 256 out of synchronization, two consecutive transmissions must be made. This situation would occur quite frequently, resulting in the customer's perception of poor performance. Furthermore, after changing the transmitter battery, the transmitter will be out of synchronization.

The security system described in US Patent 5,159,329 uses a security code section which has a fixed part and a part which varies according to a prescribed algorithm. The system advances the variable part of the code word after each transmission. The problem with such a system is that once the transmitter is operated out of range of the control unit, the transmitter and the control device are no longer synchronized. The system can be reset by a mechanical switch inside the vehicle. This situation would occur quite frequently and would perceived by the customer as poor performance.

The security system described in European Patent Specification EP-A-0 244 332 comprises a transmitter and a receiver in which the transmitted security code includes parts comprising an identification code, a count of the transmissions made by the transmitter, and a rolling code. The receiver has a memory for storing the number of transmissions received from a transmitter and will acknowledge the received security code if the transmission count in the received security code is greater than the transmission count stored in the receiver. Exact synchronization of the transmitter and receiver is therefore not necessary. It is nevertheless a requirement that the transmission count in the security code always exceeds the transmission count stored in the receiver. Should circumstances arise in which the transmitter has lost the transmission count, the system will fail because no provision is made for such a loss of count.

It would be desirable to have a remote control system that uses one-way transmissions and a rolling code and that does not depend on the synchronization of the transmitter and receiver.

The present invention now provides a receiver for use in a security system to receive and confirm a security code having parts representing an identification code, a transmission number and a rolling code; the receiver comprising:

a decoder to decode the security code and reproduce the received identification code, the transmission number and the rolling code;

an identification code memory to store a receiver identification code;

a last transmission count memory for storing a last transmission count;

a rolling code generator to update the rolling code according to a predetermined formula; and

a comparator for comparing the received identification code with the receiver identification code, for comparing the received rolling code with the refreshed rolling code, and for comparing the received transmission count with the last transmission count stored in the transmission count memory;

characterized in that the comparator is adapted to carry out the comparison of the received transmission number with the last transmission number by:

a first test step is carried out in which it is checked whether the received transmission number is greater than the last transmission number;

either by executing an enabling step to confirm the security code if the received transmission count is greater than the last transmission count; or, if this is not the case, by executing another checking step to check whether the received transmission count is less than a predetermined number; and by

a step is carried out in which the last transmission count is reset to be equal to the received transmission count - either after completion of the release step or after checking in this other check step that the received transmission count is lower than this predetermined number.

The invention further provides a method for deactivating a security system having a receiver for receiving and confirming a security code having parts representing an identification code, a transmission number and a rolling code, the method comprising:

Receiving and decoding a security code in the receiver to reproduce the received identification code, the transmission number and the rolling code;

Storing a receiver identification code in the receiver;

Storing a last transmission count in the receiver;

Refreshing the rolling code in the receiver according to a predetermined formula; and

Comparing the received identification code with the stored receiver identification code, comparing the received rolling code with the updated rolling code, and comparing the received transmission count with the last transmission count stored in the receiver;

characterized in that the comparison of the received transmission number with the last transmission number is effected by:

a first test step is carried out by checking whether the received transmission count is greater than the last transmission count;

either a release step is carried out to confirm the security code, if the received transmission count is greater than the last transmission count; or, if this is not the case, by performing another test step to check whether the received transmission count is less than a predetermined number; and by

a step is carried out in which the last transmission number is reset to be equal to the received transmission number - either after completion of the release step or after checking in this other check step that the received transmission number is lower than this predetermined number.

The invention will now be further described by way of example with reference to the accompanying drawings in which:

Figure 1 is a block diagram showing the apparatus of a preferred embodiment of the present invention;

Figure 2 is a program flow chart of a preferred method of operating the remote control system.

Referring now to Figure 1, a transmitter 20 is provided to send a security code to a receiver 40 which enables a function such as unlocking a door or turning off an alarm. The one-way signal 30 transmitted by transmitter 20 may be, for example, a radio frequency or infrared transmission using known apparatus for transmitting digital information.

When the user desires to perform a function, the security code is triggered, causing the security code to activate. Transmitter 20 has a transmission counter 22 which stores and maintains the number of transmissions (i.e., key presses) made by transmitter 20. Transmission counter 22 stores the value in a volatile memory which is erased when the transmitter battery (not shown) is removed.

Transmitter 20 also has an identification code memory 24 which stores a fixed identification code assigned to the transmitter and receiver. The identification code is stored in a non-volatile memory (e.g. ROM; read only memory) so that it is not erased if the power to transmitter 20 is interrupted.

Transmitter 20 also has a rolling code generator 26, which is based on the A non-repeating rolling code is calculated from the value of the transmission count. The calculation preferably uses a linear congruent method for generating random numbers, such as that described in "The Art of Computer Programming, Vol. 2; Semi-Numerical Algorithms" by Donald E. Knuth. The rolling code is calculated in response to the actuation of pushbutton 21. A preferred formula for calculating the rolling code is as follows:

Rolling code = ((a * transmission counter) + c) mod m

where "a" is chosen as follows:

If "m" is a power of 2, choose "a" such that (a mod 8) = 5.

If "m" is a power of ten, choose "a" such that (a mod 200) = 21.

"a" is greater than "m", preferably greater than m/100, but less than (m - SQRT m).

"c" can be any selected number that matches at least the following:

"c" is an odd number if "m" is a power of two.

"c" is not a multiple of 5 if "m" is a power of ten.

"m" should be chosen so that it is the largest possible value and does not exceed the computer word length.

The rolling code is preferably calculated accurately and without rounding errors.

The security code is a combination of the values in the transmission counter 22, identification code memory 24 and rolling code generator 26. The security code is formed and transmitted by a coding unit 28.

Receiver 40 has a decoder 42 which receives the security code generated by transmitter 20 and divides it back into the three different sections. Receiver 40 has an identification code memory 41 which has the same value as in transmitter 20 stored in a non-volatile memory. Receiver 40 also has a last transmission count memory 48 which stores the last transmission count previously sent by transmitter 20. The value in last transmission count memory 48 is not updated until a valid code is sent and the system is enabled to function.

When decoder 42 receives the security code, rolling code generator 50 calculates the rolling code based on the number of transmissions received, using the same formula as transmitter 20.

The comparator 44 then generates a signal to enable the named function if the values received by the transmitter 20 correspond to the values generated in the receiver 40 in the following manner:

1) the value in the identification code memory 41 must be equal to the received identification code; 2) the value calculated by the rolling code generator 50 must be equal to the received value of the rolling code; and 3) the value of the received transmission count must be greater than the value in the last transmission count memory 48.

Because the number of transmissions received by the transmitter only has to be greater than the value in the memory of the last transmission number 48, synchronization is not lost if the value in the counter 22 of the transmitter is increased while it is out of range of the receiver 40.

Referring now to Figure 2, a program flow chart is shown for the sequence of steps to be carried out in the receiver as a preferred method of operating the invention.

The sequence of operations begins at step 60, where the receiver waits until a transmission is detected. When a transmission is received, the security code is broken down into its three sections. Before enabling a function, the system has three separate checks. Step 62 checks whether the transmitted identification code portion (TIC) is equal to the value stored in the receiver's identification code memory. If the identification codes are not equal, step 64 is executed, which ignores the transmission as invalid and returns to the beginning of the sequence. If the identification codes are equal, step 66 is executed, which checks whether the expected rolling code (eRC) calculated by the receiver using the received transmission count value is equal to the received rolling code (rCR) calculated in the transmitter using the same mathematical function operating on the transmission count value. If the rolling codes do not match, step 64 is executed, in which the transmission is discarded as invalid, and the process begins again at step 60. If the rolling codes are equal, step 68 is executed, which checks whether the received transmission counter (rTC) value is greater than the last received transmission counter (IrTC) value stored in the receiver's memory. This step is not an equivalence to the prior art, because the transmitter may have been operated out of range of the receiver, and the transmitter's transmission counter would then be higher than that of the receiver. This eliminates the need for accurate synchronization of the transmitter and receiver. If the received transmission counter value is greater than the last transmitter value stored in the receiver's memory, the transmitted signal is considered valid and step 70 is executed, which performs a function such as opening a door or turning off an alarm. After a function is enabled in step 70, the received transmission counter value (rTC) is set in the last received transmission count (IrTC) memory in step 72.

In the event that the battery in the transmitter is changed, the transmitter memory will be lost and the transmission count value will be reset to zero. Returning to step 68, if the received transmission counter value was less than or equal to the last transmission counter value stored in the receiver's memory, step 74 is executed. Step 74 checks whether the received transmission counter value is less than a predetermined number, such as 50. If the value is 50 or higher, step 64 is executed, which passes the transmission as invalid. If the value is less than 50, step 72 is executed, in which the received transmission counter value is then placed in the receiver's memory for the last received transmission counter value. After changing the battery, the transmitter can be operated twice in the presence of the receiver to reset the value of the transmission counter in the receiver and then enable the desired function. The number 50 allows a few unintentional operations after changing the battery until the first operation within the range of the receiver.

Claims (10)

1. A receiver (40) for use in a security system to receive and confirm a security code having parts representing an identification code, a transmission number and a rolling code; the receiver comprising: a decoder (42) for decoding the security code and reproducing the received identification code, the transmission number and the rolling code; an identification code memory (41) for storing a receiver identification code; a last transmission count memory (48) for storing a last transmission count; a rolling code generator (50) for updating the rolling code according to a predetermined formula; and a comparator (44) for comparing the received identification code with the receiver identification code, for comparing the received rolling code with the updated rolling code, and for comparing the received transmission number with the last transmission number stored in the transmission number memory (48); characterized in that the comparator (44) is adapted to carry out the comparison of the received transmission number with the last transmission number by: a first test step (68) is carried out in which it is checked whether the received transmission number is greater than the last transmission number; either an enabling step (70) is carried out to confirm the security code if the received transmission number is greater than the last transmission number; or if this is not the case, another checking step (74) is carried out to check whether the received transmission number is lower than a predetermined number; and a step (72) is carried out in which the last transmission number is reset to be equal to the received transmission number - either after completion of the release step (70) or after checking in this other check step (74) that the received transmission number is lower than this predetermined number. 2. A receiver according to claim 1, wherein the predetermined number is 50. 3. A receiver according to claim 1 or 2, wherein the rolling code generator (50) calculates the rolling code according to a predetermined formula based on the last transmission count stored in the last transmission count memory (48). 4. A receiver according to claim 1, 2 or 3, in which the predetermined formula generates a rolling code according to the following relationship: Rolling code = ((a * transmission counter) + c) mod m where "a" is chosen as follows: If "m" is a power of two: (a mod 8) = 5; If "m" is a power of ten: (a mod 200) = 21; "a" is greater than "m" but less than (m - SQRT m); "c" is chosen as an odd number if "m" is a power of two; and not as a multiple of 5 if "m" is a power of ten; and "m" is the largest possible value so that the computer word length is not exceeded. 5. A security system comprising a receiver (40) according to claim 1, 2, 3 or 4 and a transmitter (20) for transmission to the receiver, the transmitter (20) including: a transmission counter (22) for counting the number of transmissions made by the transmitter (20); an identification code memory (24) for storing an identification code assigned to the transmitter (20); a rolling code generator (26) for calculating a rolling code using the predetermined formula; and an encoding mechanism (28) for forming and transmitting a security code; having parts representing the identification code attributed to the transmitter, the rolling code and the number of transmissions made by the transmitter. 6. A security system according to claim 5, in which the transmission counter (22) has a volatile memory for storing the number of transmissions made by the transmitter (20). 7. A method for deactivating a security system having a receiver (40) for receiving and confirming a security code which having parts representing an identification code, a transmission number and a rolling code; the method comprising: Receiving and decoding a security code in the receiver to reproduce the received identification code, the transmission number and the rolling code; Storing a receiver identification code in the receiver; Storing a last transmission count in the receiver; Refreshing the rolling code in the receiver according to a predetermined formula; and Comparing the received identification code with the stored receiver identification code, comparing the received rolling code with the updated rolling code, and comparing the received transmission count with the last transmission count stored in the receiver; characterized in that the comparison of the received transmission number with the last transmission number is effected by: a first check step (68) is carried out by checking whether the received transmission number is greater than the last transmission number; either an enabling step (70) is carried out to confirm the security code if the received transmission number is greater than the last transmission number; or if this is not the case, another checking step (74) is carried out to check whether the received transmission number is lower than a predetermined number; and a step (72) is carried out in which the last transmission number is reset to be equal to the received transmission number - either after completion of the release step (70) or after checking in this other check step (74) that the received transmission number is lower than this predetermined number. 8. A method according to claim 7, wherein the predetermined number is 50. 9. A method according to claim 7 or 8, in which the rolling code is generated according to a predetermined formula based on the last transmission number stored in the receiver. 10. A method according to claim 7, 8 or 9, wherein the predetermined formula generates a rolling code according to the following relationship: Rolling code = ((a * transmission counter) + c) mod m where "a" is chosen as follows: If "m" is a power of two: (a mod 8) = 5; If "m" is a power of ten: (a mod 200) = 21; "a" is greater than "m" but less than (m - SQRT m); "c" is chosen as an odd number if "m" is a power of two; and not as a multiple of 5 if "m" is a power of ten; and "m" is the largest possible value so that the computer word length is not exceeded.